The present invention relates to a communications control unit for scanning a number of lines and for transmitting and receiving data on a time-sharing basis; and, more particularly, to a communications control unit for efficiently transmitting and receiving data on full-duplex lines.
Hitherto, as a system for transmitting and receiving data by scanning a plurality of lines, there has been known a line scanning system disclosed in JP-No.-B-55-3743 or the like. According to this system, the information bits which are transferred from each line are assembled to construct one character, the constructed character is set into an assembly buffer area in a line control word memory which is provided in correspondence to each line and upon completion of the construction of one character, a flag bit indicative of the completion of the construction of one character is also set into this line control word. On the other hand, a line controller scans the flag bit in the line control word memory for every line. For the line in which a character is not constructed yet, only the flag bit is checked and the next line is then scanned. With respect to the line in which the character has already been constructed, if this character should be transferred to an upper data processor, it is transferred and thereafter, the next line is scanned. The transmission is executed by operations which are opposite to the receiving operations mentioned above. Namely, the line controller scans the line control word memory for every line and checks for a flag bit indicating that the character which had previously been transferred to the buffer area in the line control word memory has been transferred to the line. If this flag bit is set and the data to be subsequently transmitted exists, the next character is transferred. Thereafter, the next line is examined.
According to this system, in the case of the half-duplex line, only either the reception or the transmission is executed at one time by use of one line control word. By alternately performing the reception control and transmission control, the transmitting and receiving operations of data are executed.
In the foregoing conventional technique, in the case of the full-duplex line, the line is handled as two half-duplex lines in such a manner that two line control words are used and two time slots are taken for examining the line, thereby executing the transmitting/receiving operations. Namely, the line control word for reception is checked by one time slot and the line control word for transmission is examined by the other time slot. Therefore, the processing time for the full-duplex line is twice as long as that for the half-duplex line. Thus, in the case of supporting a high speed line by this line controller, there is a problem that assuming that the line speeds are the same, the full-duplex lines as many as only half of the number of half-duplex lines can be processed.
On the other hand, in the case of controlling the transmitting side depending on the state of the receiving side, there is a problem that a special hardware to communicate between the receiving side and the transmitting side is necessary.